The SWI-Prolog engine uses three stacks the local stack
(also called environment stack) stores the environment frames
used to call predicates as well as choice points. The
global stack (also called heap) contains terms,
floats, strings and large integers. Finally, the trail stack
records variable bindings and assignments to support
backtracking. Except for available memory, there is no
hard limit for the sizes of the stacks.42As
of version 9.3.6. Older versions have a hard limit on 32-bit hardware of
128Mb for each stack.
The combined stack size (per thread) has a soft limit
implemented by the writeable flag stack_limit
or the command line option --stack-limit. Currently the
default limit is 1Gb. Considering portability, applications that need to
modify the default limits are advised to do so using the Prolog flag stack_limit.
| Area name | Description |
| local stack | The local stack is used
to store the execution environments of procedure invocations. The space
for an environment is reclaimed when it fails, exits without leaving
choice points, the alternatives are cut off with the !/0 predicate or no
choice points have been created since the invocation and the last
subclause is started (last call optimisation). |
| global stack | The global stack is used to store
terms, strings, big integers, rational numbers and floating numbers
created during Prolog's execution. Data on this stack is reclaimed by
backtracking to a point before the data was created or by garbage
collection (provided the data is no longer referenced). |
| trail stack | The trail stack is used to store
assignments during execution. Entries on this stack remain alive until
backtracking before the point of creation or the garbage collector
determines they are no longer needed.
As the trail and global stacks are garbage collected together, a
small trail can cause an excessive amount of garbage collections. To
avoid this, the trail is automatically resized to be at least 1/6th of
the size of the global stack. |
Table 2 : Memory areas
With
the heap, we refer to the memory area used by malloc() and
friends. SWI-Prolog uses the area to store atoms, functors, predicates
and their clauses, records and other dynamic data. No limits are imposed
on the addresses returned by malloc() and friends.
